Method of producing columnar crystal texture in sintered permanent magnets



United States Patent 3,219,496 METHOD OF PRODUCING COLUMNAR CRYS- TAL TEXTURE 1N SINTERED PERMANENT MAGNETS Erich A. Steingriiver, Bonn, and Giinther F. A. Heimke,

Buschdorf, near Bonn, Germany, assignors to Magnetfabrik Bonn Gewerkschaft Windhorst, Bonn, Germany No Drawing. Filed Feb. 12, 1963, Ser. No. 257,835 Claims priority, application Germany, Feb. 17, 1962, M 51,856 6 Claims. (Cl. 148105) This invention relates to sintered metallic permanent magnets.

The invention is particularly concerned with magnets of the alnico type of the approximate composition of 10 to 40% Co, 10 to 22% Ni, to 14% Al, 0 to Cu, 0 to 10% Ti, 0 to 3% Si, the balance being Fe and not more than 1% of the conventional additives and/or impurities. It is known to prepare such magnets having a crystallographic texture. Such texture is due to the fact that one direction of the crystal edge [100] is oriented in parallel direction, the other crystal edges being evenly distributed in the two other spatial directions. The magnetic preferred direction, produced by a heat treatment in a magnetic field, as well as the direction of magnetization in the use of magnets coincide with the [100-]-direction common to all the crystallites. When said texture is present, the magnets have been designated as magnets having columnar crystallization or columnar growth, whereby the [100]-direction, which is common to all the crystallites, is designated the columnar axis.

In sintered permanent magnets, the following procedures have been available for producing said texture:

(a) Monocrystals or preformed columns are manually embedded correctly oriented as nuclei into the mixture of the metal powders forming the alloy;

(b) Monocrystal grains of the same or a similar alloy, obtained in some other way, are incorporated into the mixture of the alloy-forming metal powders.

Prior to being compressed to the ultimate shape, said mixture is subjected to a magnetic field which orients the alnico-monocrystal particles with one of their [1001-directions parallel to the magnetic field. Said oriented crystallites serve then again as crystallization nuclei.

to) Wires of one of the alloying components are embedded into the mixture of the alloy-forming metal powders so as to determine the ultimate columnar axis by the wire axis. The amount of the component incorporated in wire form is to be taken into account when making up the mixture of the metal powders.

((1) Finally, it has been proposed to produce a preferred crystallization in sintered permanent magnets of the type here described by producing a gradient of concentration of one or more of the alloying components in the powdery mixture within the body to be sintered. The thus obtained preferred spatial direction in said body may produce preferred crystallization in one direction and thereby formation of a column.

All said procedures have the drawback to require additional operations in the manufacture of the pressed bodies which slow up the production.

The principal object of the present invention is to provide a method which avoids said drawbacks.

Other objects and advantages will be apparent upon consideration of the specification and claims.

According to the invention, the columnar crysallization is produced by means of a solid state reaction in the finished sintered body as a secondary recrystallization.

Sintered bodies are very fine-grained and contain a large number of small pores. In converting such a texture into a structure of oriented crystallization, a very great num- 3,219,496 Patented Nov. 23, 1965 Ice her of grain boundaries would have to be eliminated or displaced. It was, therefore, not to be expected that a simple secondary recrystallization would produce oriented columnar crystallization.

Compared with the procedures (a) to (d), recited hereinabove, our novel method has the advantage that no modifications are required in the production process of the magnets including the sintering operation. It is only necessary to heat the finished sintered bodies to the recrystallization temperature. For this purpose, they are moved parallel to the desired direction of the columnar axes through a temperature range extending from a temperature immediately below their melting point to a temperature of C. below said melting point. Subsequently, they are moved, still parallel to the desired direction of the columnar crystallization, into a low temperature zone which produces inside the sintered bodies a temperature gradient parallel to their direction of movement and to the desired direction of the columnar axes which gradient should be at least =10 C./cm., preferably =20 C./cm.

The recrystallizing operation may be combined with the conventional heat treatment in a magnetic field; in this way, producing a columnar crystallization in accordance with our method requires only a small additional heat treatment of the magnets.

The heating to recrystallization temperature may be carried out by means of an induction coil, by heat conduction, by heat radiation, or by means of electron or high energy ion beams.

The following examples are given to illustrate the method of the invention.

Example 1 Conventionally prepared sintered round alnico magnets having a diameter of 12 mm. and a height of 10 mm. were stacked upon each other. They were held together by means of a clamp and dipped into a mixture of finely ground quartz and ethyl silicate. The mass adhering to the surface of the magnet cylinder was dried and the procedure was repeated until a layer of about 7 mm. thickness had been deposited. Then the clamp was removed, and only a hook was left at the upper end.

The thus obtained ceramic coat was baked by a heat treatment of 1 hour at 1200 C. in a muffle furnace whereby the heating up and cooling in the furnace was carried out together with the heating and cooling of the furnace.

In this way, ceramic tubes were obtained which contained the magnets and were suspended from the hook left at the upper end. The tube was inserted in a water cooled medium frequency coil heated with 3 kw. at 10,000 Hz.

After the heat was turned on, the lower end of the cylinder was heated to a temperature closely below the melting point of the alnico, which temperature was generally between 1390 and 1430 C. When said temperature had been reached at the lower end of the cylinder ceramic tube, it was lowered at a rate of about 0.5 mm./sec. By adjusting the energy supplied to the coil, the temperature inside the coil was maintained at about 1400 C.

After the lower end of the ceramic tube containing the magnets had emerged from the lower end of the coil, it was placed on a water cooled chilling plate of copper of the same diameter. At the same time, a tubular furnace arranged below the medium frequency coil coaxially therewith was heated at about 1300 C. The chilling plate and ceramic tube were lowered through the tubular furnace while maintaining the above recited rate of speed.

After cooling, the ceramic tube was broken, and the magnets were subjected to the conventional heat and magnetizing treatment. Care was taken to arrange the magnetic field producing the preferential direction of magnetization parallel to the cylinder axis of the magnets.

After completed treatment, the (BH) values in the direction parallel to the cylinder axis were 6 to 7x10 gauss/oersted.

Example 2 Magnets of the same dimensions as in Example 1 were stacked in a ceramic tube of 12.5 mm. inner and 16 mm. outer diameter to a height of 350 mm. The upper end of the tube was provided with a hook to manipulate the tube. At the lower end, it had an inner collar of 10 mm. diameter to prevent the magnets from dropping out. The chilling plate applied after passage of the tube through the medium frequency coil had also a diameter of 10 mm. and could, therefore, be placed through said collar into direct contact with the lowermost magnet.

The procedure was the same as described in Example 1 except that the magnets could be recovered without breaking the ceramic tube. The (BH) values were between 5.8 and 7.1 10 gauss/oersted.

Example 3 The test of Example 1 was repeated but the ceramic tube was not broken but carefully blown with compressed air until it had a temperature of about 900 C.; subsequently, it was placed for cooling into a cylindrical winding in which a magnetic field of 2000 oersted was maintained. In this manner, the magnetic field treatment was applied immediately following the recrystallization treatment without intermediate cooling of the magnets to room v temperature. The further heat treatment was the conventional one. The obtained magnets had (BI-I) values in the preferential direction of 6.3 to 7.4 10

Example 4 In this example, test 2 was repeated but with the modification described in Example 3. The (BH), values were 6.2 to 7.5 10

The conventional heat treatment referred to above consisted normally in heating the magnets to 1280 C. and cooling in a magnetic field of about 1500 oersted at 820 C., cooling in said field during 30 minutes to about 600 C., then cooling to room temperature, and finally a twostage annealing first 2 hours at 630 C. and then for 6 hours at 560 C., followed by cooling to room temperature.

We claim:

1. A method for producing a columnar crystal texture in sintered permanent magnets of the alnico type comprising heating a sintered alnico body to a temperature close to the melting point, passing the body through a zone where its temperature is lowered by about 100 0, thereby producing secondary recrystallization, and establishing in said body in the direction of its passage through said zone to a temperature gradient of 10 to 100 C./ cm. so as to produce oriented crystal growth in said direction.

2. The method claimed in claim 1 wherein said temperature gradient is about 20 C./cm.

3. The method as claimed in claim 1 wherein said body passes said temperature zone within 4 to 20 seconds.

4. The method as claimed in claim 3 wherein said body passes said temperature zone within about 10 seconds.

5. The method as claimed in claim 1 wherein said body leaves said temperature zone with a temperature of about 1300 C. and is then quenched to about 900 C.

6. The method as claimed in claim 1 comprising subjecting said body, immediately after it has left said temperature zone, to a heating treatment in a magnetic field.

References Cited by the Examiner UNITED STATES PATENTS 1,738,307 12/1929 McKeehan 148-122 XR 2,084,133 6/1937 Dixon et al 148108 2,102,683 12/1937 Dixon 148122 2,295,082 9/1942 Jonas 148103 2,617,723 11/1952 Stndders et al 148-16 OTHER REFERENCES Magnetic Properties of Metals and Alloys, ASM, Cleveland, Ohio, 1959, pages 306 and 307.

Metallurgy and Magnestism, ASM, Cleveland, Ohio, 1949, page 65.

HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner. 

1. A METHOD FOR PRODUCING A COLUMNAR CRYSTAL TEXTURE IN SINTERED PERMANENT MAGNETS OF THE ALNICO TYPE COMPRISING HEATING A SINTERED ALNICO BODY TO A TEMPERATURE CLOSE TO THE MELTING POINT, PASSING THE BODY THROUGH A ZONE WHERE ITS TEMPERATURE IS LOWERED BY ABOUT 100*C., THEREBY PRODUCING SECONDARY RECRYSTALLIZATION, AND ESTABLISHING IN SAID BODY IN THE DIRECTION OF ITS PASSAGE THROUGH SAID ZONE TO A TEMPERATURE GRADIENT OF 10 TO 100*C./CM. SO AS TO PRODUCE ORIENTED CRYSTAL GROWTH SAID DIRECTION. 